/*
 * YAFFS: Yet Another Flash File System. A NAND-flash specific file system.
 *
 * Copyright (C) 2002-2011 Aleph One Ltd.
 *   for Toby Churchill Ltd and Brightstar Engineering
 *
 * Created by Charles Manning <charles@aleph1.co.uk>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

/*
 * This code implements the ECC algorithm used in SmartMedia.
 *
 * The ECC comprises 22 bits of parity information and is stuffed into 3 bytes.
 * The two unused bit are set to 1.
 * The ECC can correct single bit errors in a 256-byte page of data. Thus, two
 * such ECC blocks are used on a 512-byte NAND page.
 *
 */

#include "yportenv.h"

#include "yaffs_ecc.h"

/* Table generated by gen-ecc.c
 * Using a table means we do not have to calculate p1..p4 and p1'..p4'
 * for each byte of data. These are instead provided in a table in bits7..2.
 * Bit 0 of each entry indicates whether the entry has an odd or even parity,
 * and therefore this bytes influence on the line parity.
 */

static const unsigned char column_parity_table[] = {
        0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
        0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
        0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
        0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
        0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
        0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
        0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
        0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
        0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
        0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
        0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
        0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
        0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
        0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
        0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
        0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
        0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
        0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
        0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
        0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
        0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
        0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
        0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
        0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
        0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
        0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
        0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
        0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
        0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
        0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
        0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
        0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
};


/* Calculate the ECC for a 256-byte block of data */
void yaffs_ecc_calc(const unsigned char *data, unsigned char *ecc)
{
        unsigned int i;
        unsigned char col_parity = 0;
        unsigned char line_parity = 0;
        unsigned char line_parity_prime = 0;
        unsigned char t;
        unsigned char b;

        for (i = 0; i < 256; i++) {
                b = column_parity_table[*data++];
                col_parity ^= b;

                if (b & 0x01) { /* odd number of bits in the byte */
                        line_parity ^= i;
                        line_parity_prime ^= ~i;
                }
        }

        ecc[2] = (~col_parity) | 0x03;

        t = 0;
        if (line_parity & 0x80)
                t |= 0x80;
        if (line_parity_prime & 0x80)
                t |= 0x40;
        if (line_parity & 0x40)
                t |= 0x20;
        if (line_parity_prime & 0x40)
                t |= 0x10;
        if (line_parity & 0x20)
                t |= 0x08;
        if (line_parity_prime & 0x20)
                t |= 0x04;
        if (line_parity & 0x10)
                t |= 0x02;
        if (line_parity_prime & 0x10)
                t |= 0x01;
        ecc[1] = ~t;

        t = 0;
        if (line_parity & 0x08)
                t |= 0x80;
        if (line_parity_prime & 0x08)
                t |= 0x40;
        if (line_parity & 0x04)
                t |= 0x20;
        if (line_parity_prime & 0x04)
                t |= 0x10;
        if (line_parity & 0x02)
                t |= 0x08;
        if (line_parity_prime & 0x02)
                t |= 0x04;
        if (line_parity & 0x01)
                t |= 0x02;
        if (line_parity_prime & 0x01)
                t |= 0x01;
        ecc[0] = ~t;

}

/* Correct the ECC on a 256 byte block of data */

int yaffs_ecc_correct(unsigned char *data, unsigned char *read_ecc,
                      const unsigned char *test_ecc)
{
        unsigned char d0, d1, d2;       /* deltas */

        d0 = read_ecc[0] ^ test_ecc[0];
        d1 = read_ecc[1] ^ test_ecc[1];
        d2 = read_ecc[2] ^ test_ecc[2];

        if ((d0 | d1 | d2) == 0)
                return 0;       /* no error */

        if (((d0 ^ (d0 >> 1)) & 0x55) == 0x55 &&
            ((d1 ^ (d1 >> 1)) & 0x55) == 0x55 &&
            ((d2 ^ (d2 >> 1)) & 0x54) == 0x54) {
                /* Single bit (recoverable) error in data */

                unsigned byte;
                unsigned bit;

                bit = byte = 0;

                if (d1 & 0x80)
                        byte |= 0x80;
                if (d1 & 0x20)
                        byte |= 0x40;
                if (d1 & 0x08)
                        byte |= 0x20;
                if (d1 & 0x02)
                        byte |= 0x10;
                if (d0 & 0x80)
                        byte |= 0x08;
                if (d0 & 0x20)
                        byte |= 0x04;
                if (d0 & 0x08)
                        byte |= 0x02;
                if (d0 & 0x02)
                        byte |= 0x01;

                if (d2 & 0x80)
                        bit |= 0x04;
                if (d2 & 0x20)
                        bit |= 0x02;
                if (d2 & 0x08)
                        bit |= 0x01;

                data[byte] ^= (1 << bit);

                return 1;       /* Corrected the error */
        }

        if ((hweight8(d0) + hweight8(d1) + hweight8(d2)) == 1) {
                /* Reccoverable error in ecc */

                read_ecc[0] = test_ecc[0];
                read_ecc[1] = test_ecc[1];
                read_ecc[2] = test_ecc[2];

                return 1;       /* Corrected the error */
        }

        /* Unrecoverable error */

        return -1;

}

/*
 * ECCxxxOther does ECC calcs on arbitrary n bytes of data
 */
void yaffs_ecc_calc_other(const unsigned char *data, unsigned n_bytes,
                          struct yaffs_ecc_other *ecc_other)
{
        unsigned int i;
        unsigned char col_parity = 0;
        unsigned line_parity = 0;
        unsigned line_parity_prime = 0;
        unsigned char b;

        for (i = 0; i < n_bytes; i++) {
                b = column_parity_table[*data++];
                col_parity ^= b;

                if (b & 0x01) {
                        /* odd number of bits in the byte */
                        line_parity ^= i;
                        line_parity_prime ^= ~i;
                }

        }

        ecc_other->col_parity = (col_parity >> 2) & 0x3f;
        ecc_other->line_parity = line_parity;
        ecc_other->line_parity_prime = line_parity_prime;
}

int yaffs_ecc_correct_other(unsigned char *data, unsigned n_bytes,
                            struct yaffs_ecc_other *read_ecc,
                            const struct yaffs_ecc_other *test_ecc)
{
        unsigned char delta_col;        /* column parity delta */
        unsigned delta_line;    /* line parity delta */
        unsigned delta_line_prime;      /* line parity delta */
        unsigned bit;

        delta_col = read_ecc->col_parity ^ test_ecc->col_parity;
        delta_line = read_ecc->line_parity ^ test_ecc->line_parity;
        delta_line_prime =
            read_ecc->line_parity_prime ^ test_ecc->line_parity_prime;

        if ((delta_col | delta_line | delta_line_prime) == 0)
                return 0;       /* no error */

        if (delta_line == ~delta_line_prime &&
            (((delta_col ^ (delta_col >> 1)) & 0x15) == 0x15)) {
                /* Single bit (recoverable) error in data */

                bit = 0;

                if (delta_col & 0x20)
                        bit |= 0x04;
                if (delta_col & 0x08)
                        bit |= 0x02;
                if (delta_col & 0x02)
                        bit |= 0x01;

                if (delta_line >= n_bytes)
                        return -1;

                data[delta_line] ^= (1 << bit);

                return 1;       /* corrected */
        }

        if ((hweight32(delta_line) +
             hweight32(delta_line_prime) +
             hweight8(delta_col)) == 1) {
                /* Reccoverable error in ecc */

                *read_ecc = *test_ecc;
                return 1;       /* corrected */
        }

        /* Unrecoverable error */

        return -1;
}